1. Field of the Invention
The present invention relates to a control method for a mechanical shutter such as a focal plane shutter apparatus for use in a camera, and to an image sensing apparatus equipped with the mechanical shutter.
2. Description of the Related Art
Heretofore, a direct holding focal plane shutter is known that holds a shutter charged state by energizing electromagnets corresponding respectively to the first and second curtains of the shutter, and performs exposure by successively ceasing to energize the electromagnets (e.g., see Japanese Utility Model Publication No. 6-26895).
Japanese Patent Laid-Open No. 2005-283897 discloses a direct holding focal plane shutter that varies the applied voltage to the electromagnets to achieve power reduction.
Also, Japanese Patent Laid-Open No. 2001-215555 and Japanese Patent Laid-Open No. 2001-23220 disclose observing the object with a monitor such as an LCD in a camera with an image sensor such as a CDD or a CMOS sensor, by holding the second curtain with the first curtain opening the shutter (hereinafter, “electronic viewfinder (EVF) function”).
Further, there are single lens reflex (SLR) digital cameras that perform the image sensing operation using both a focal plane shutter (hereinafter, “mechanical shutter”) and an electronic shutter (e.g., see Japanese Patent Laid-Open No. 11-41523 (par. 0044-0050, FIGS. 1-3, etc.)). With this shutter, the second curtain is constituted by a mechanical shutter, and the reset scan of the image sensor is performed using the electronic shutter before the second curtain travels (hereinafter, “hybrid shutter”). The reset scan of the image sensor (in other words, charge accumulation start scan) is performed using a scan pattern adjusted to the traveling characteristics of the second curtain mechanical shutter (e.g., see Japanese Patent Laid-Open No. 2001-23220).
However, the following problems arise when a focal plane shutter such as disclosed by Japanese Utility Model Publication No. 6-26895 is used in an image sensing apparatus with an EVE function such as disclosed by Japanese Patent Laid-Open No. 2001-215555 and Japanese Patent Laid-Open No. 2001-23220. That is, not only is power wasted due to having to keep the electromagnet holding the second curtain energized during monitor display, but the departure timing of the electromagnets varies as a result of the electromagnets becoming heated, reducing exposure accuracy.
While use of a focal plane shutter such as disclosed in Japanese Patent Laid-Open No. 2005-283897 suppresses heating of the electromagnets, the shutter release timing varies when performing high speed image sensing, because the holding power of the electromagnets is unstable at low voltages. Consequently, using the focal plane shutter control method of Japanese Patent Laid-Open No. 2005-283897 is desirable when performing relatively low speed image sensing at which the variation in shutter release timing is virtually negligible. Since there is little room time-wise to switch voltages with high speed image sensing, Japanese Patent Laid-Open No. 2005-283897 can actually only be applied to low speed image sensing.
An image sensing apparatus that has a reflective mirror for directing incident light from the image sensing lens to a viewfinder optical system on the front of a focal plane shutter, enabling the object to be observed through a viewfinder, as with an SLR camera, will be considered next. Here, the image sensing apparatus is assumed to be able to perform both ordinary image sensing that uses both of the first and second curtains of a mechanical shutter, and hybrid shutter image sensing using an electronic first curtain and a mechanical shutter second curtain during EVF execution.
At this time, with high speed image sensing, voltage control such as shown in FIG. 12 is desired over voltage reduction in the case of ordinary image sensing for the foregoing reasons. On the other hand, with image sensing during EVF execution, a drive method such as disclosed in Japanese Patent Laid-Open No. 2005-283897 desirably is applied in order to reduce power consumption and suppress coil heating (time chart shown in FIG. 14). However, when the power reduction shown in FIG. 14 is performed for image sensing while using the EVF, the variation in shutter release timing increases, and a difference also appears in exposure between image sensing using the optical viewfinder and using the EVF. This occurs for the following reasons.
Since the electromagnets produces the self-induced electromotive force, the armature and the yoke do not depart immediately after cutting coil voltage, producing a time lag before departure as shown in FIG. 15. This time lag becomes longer as voltages increase, and becomes shorter as voltages decrease, as indicated by TH and TL in FIG. 15. Thus, a difference in exposure arises between image sensing without the EVF and image sensing with the EVF in which voltage control such as shown in FIG. 14 is performed, even at the same fast shutter speed (i.e., even when T1 in FIGS. 12 and 14 is the same).